Immunoconjugates consisting of antibody joined to a cytotoxic agent have been used in attempts to achieve selective killing of particular target cells, such as tumor cells. In theory, immunoconjugates or targeting protein conjugates should effect specific cellular cytotoxicity. In practice, however, in vivo administration of immunoconjugates has proven less efficacious than anticipated.
Several disadvantages related to retention, internalization and translocation of immunoconjugates have been identified. For instance, optimal retention of isotope-antibody fragment conjugates within tumor tissue after in vivo administration has not been demonstrated. Additional problems associated with target cell internalization and translocation of immunoconjugates have been recognized, particularly in regards to translocation and internalization of A-chain (derived from plant or bacterial toxin) immunoconjugates.
Thus, there is a need in the art for improved: (1) retention of targeting protein conjugates (especially antibody fragment conjugates) at target cell plasma membranes; (2) internalization of targeting protein conjugates into target cell endosomic vesicles; and (3) translocation of targeting protein conjugates across target cell endosomic vesicular membranes into the cytoplasm. Enhancement of the interaction of targeting protein conjugates with plasma membranes and/or internal membranes of target cells may improve the cytotoxicity of targeting protein conjugates administered in vivo. The present invention fulfills this need and further provides other related advantages.